Battery charging top-off

ABSTRACT

This disclosure describes techniques for a method of charging a battery. In an example, the method includes determining a capacity of the battery, and determining a state of charge of the battery. The method also includes charging the battery with a charging current, wherein the charging current is based on the capacity of the battery and the state of charge of the battery, and wherein charging the battery comprises reducing the charging current in response to the state of charge of the battery changing with respect to the capacity of the battery.

TECHNICAL FIELD

This disclosure is directed to techniques for charging a battery.

BACKGROUND

Devices often make use of one or more rechargeable or non-rechargeablepower sources, such as batteries, to provide operating power tocircuitry of the device. During operation, the charge level of a powersource drops due to power consumption by the device. The device mayprovide some indication of remaining charge as the power source drains,e.g., as the battery or batteries drain. A user of the device mayutilize the remaining charge indication to determine whether the powersource needs to be replaced or recharged. By replacing or recharging thepower source before the charge on the power source is fully depleted,the user can ensure that operation of the device will not beinterrupted, or otherwise adversely impacted, due to power sourcedepletion.

SUMMARY

In general, this disclosure describes techniques for charging a battery,such as a rechargeable battery in an implantable medical device. Forexample, techniques of this disclosure include charging a battery duringa top-off period, which typically occurs during an end period of acharging session (e.g., as the battery approaches full charge). Duringcharging, an impedance of the battery may increase as the batteryreaches full charge. To avoid a corresponding rise in voltage, which maycause the battery to swell, a charging current may be reduced during atop-off period. In this way, the battery may be fully charged withoutincreasing the voltage of the battery in a potentially undesirable way.

Certain techniques of this disclosure relate to charging a batteryduring a top-off period based on a capacity of the battery and a stateof charge of the battery. For example, according to the techniques ofthis disclosure, a capacity of a battery may be determined based on anamount of charge that is consumed during battery discharge. That is,assuming the battery is at full capacity prior to discharge, thecapacity of the battery may be determined based on a difference betweenthe initial, full capacity and an amount of charge that is consumedduring discharge. When recharging the battery, the battery state ofcharge may be monitored relative to the determined capacity of thebattery. As the state of charge increases toward the capacity of thebattery, a current used to charge the battery may be decreased. That is,according to the aspects of this disclosure, the current used to chargethe battery during a top-off period toward the end of a charging sessionmay be decreased based on the state of charge of the battery.

Certain techniques of this disclosure also relate to charging a batteryduring a top-off period until a predetermined charging terminationparameter is reached. According to aspects of this disclosure, thecharging termination parameter for a current charging session may bedetermined based on a duration of a previous charging session.Additionally or alternatively, in some examples, the chargingtermination parameter for the current charging session may be determinedbased on an amount of charge that was administered to the battery duringthe previous charging session, or a combination of both the duration andamount of charge. For example, the techniques of this disclosure includeadapting the charging termination parameter (e.g., lowering or raisingthe value) based on a duration of the previous charging session and/oran amount of charge that was administered during the previous chargingsession. In some examples, an impedance of a battery may increase overtime, which may require a longer charging session to reach the chargingtermination parameter. The techniques of this disclosure may be used toaccount for changing battery impedance, while also maintaining a targetcharging time.

In one example, aspects of this disclosure relate to a method ofcharging a battery. The method includes determining a capacity of thebattery; determining a state of charge of the battery; and charging thebattery with a charging current, wherein the charging current is basedon the capacity of the battery and the state of charge of the battery,and wherein charging the battery comprises reducing the charging currentin response to the state of charge of the battery changing with respectto the capacity of the battery.

In another example, aspects of this disclosure relate to an implantablemedical device (IMD) comprising a battery configured to power the IMD; amemory storing instructions; and one or more processors configured toexecute the instructions. Upon execution of the instructions, the one ormore processors cause determining a capacity of the battery; determininga state of charge of the battery; and charging the battery with acharging current, wherein the charging current is based on the capacityof the battery and the state of charge of the battery, and whereincharging the battery comprises reducing the charging current in responseto the state of charge of the battery changing with respect to thecapacity of the battery.

In another example, aspects of this disclosure relate to an apparatusfor charging a battery that includes means for determining a capacity ofthe battery; means for determining a state of charge of the battery; andmeans for charging the battery with a charging current, wherein thecharging current is based on the capacity of the battery and the stateof charge of the battery, and wherein charging the battery comprisesreducing the charging current in response to the state of charge of thebattery changing with respect to the capacity of the battery.

The details of one or more aspects of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the disclosure will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example therapy systemthat includes an implantable stimulator coupled to a stimulation lead.

FIG. 2 is a conceptual diagram illustrating another example therapysystem that includes an implantable stimulator coupled to a stimulationlead.

FIG. 3 is a block diagram illustrating various example components of animplantable electrical stimulator.

FIG. 4 is a block diagram illustrating various example components of anexternal programmer for an implantable electrical stimulator.

FIG. 5 is a graph illustrating an example charging session having atop-off period.

FIG. 6 is a graph illustrating another example charging session havinganother top-off period.

FIG. 7 is a flow chart illustrating an example method of recharging abattery according to aspects of this disclosure.

FIG. 8 is a graph illustrating an example charging session having atop-off period.

FIG. 9 is a graph illustrating another example charging session havinganother top-off period.

FIG. 10 is a graph illustrating another example charging session havinganother top-off period.

FIG. 11 is a flow chart illustrating an example method of recharging abattery according to aspects of this disclosure.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for charging a battery.For example, techniques of this disclosure include charging a batteryduring a top-off period, which typically occurs during the end of acharging session (e.g., as the battery approaches full charge). That is,during charging, an impedance of the battery may increase as the batteryreaches full charge. To avoid a corresponding rise in voltage, which maycause the battery to swell, a charging current may be reduced during atop-off period. In this way, the battery may be fully charged withoutincreasing the voltage of the battery in a potentially adverse way.

Certain techniques of this disclosure relate to charging a batteryduring a top-off period based on a capacity of the battery and a stateof charge of the battery. For example, according to the techniques ofthis disclosure, a capacity of a battery may be determined based on anamount of charge that is consumed during battery discharge. That is,assuming the battery is at full capacity prior to discharge, thecapacity of the battery may be determined based on a difference betweenthe initial, full capacity and an amount of charge that is consumedduring discharge. When recharging the battery, the battery state ofcharge may be monitored relative to the determined capacity of thebattery. As the state of charge increases toward the capacity of thebattery, a current used to charge the battery may be decreased. That is,according to the aspects of this disclosure, the current used to chargethe battery during a top-off period toward the end of a charging sessionmay be decreased based on the state of charge of the battery.

Certain techniques of this disclosure also relate to charging a batteryduring a top-off period until a charging termination parameter value isreached. The charging termination parameter may include, for example, aminimum charging current cut off value. The minimum charging current cutoff value may be a minimum current that is allowed for charging. Thecharging termination parameter may additionally or alternatively includea power cut off value. For example, a power level cut off value may beapproximately equivalent to a minimum power level at which the batterymay be charged (e.g., a minimum output power of a programmer or otherrecharger charging the battery). In still other examples, other chargingtermination parameters may be used, such as other parameters thatindicate when a battery has reached full charge (e.g., batteryimpedance, and the like).

For example, in an example in which the charging termination parameteris a minimum charging current cut off, a current used to charge thebattery may be reduced until a charging current cut off value is reached(e.g., a minimum current that is allowed for charging). According toaspects of this disclosure, the charging current cut off value for acurrent charging session may be determined based on a duration of aprevious charging session and/or amount of charge that was administeredto the battery during the previous charging session. For example, thetechniques of this disclosure include adapting the charging current cutoff value (e.g., lowering or raising the value) based on a duration ofthe previous charging session and/or an amount of charge that wasadministered during the previous charging session. In some examples, animpedance of a battery may increase over time, which may require alonger charging session to reach the charging current cut off value.Accordingly, the techniques of this disclosure may allow a chargingsession to be maintained for a certain duration, while also stopping thecharging session prior to the end of the duration if the chargingcurrent is too low. While described with respect to a minimum chargingcurrent cut off value, it should be understood that other chargingtermination parameters (e.g., power level, impedance, and the like) maybe similarly adjusted according to the techniques of this disclosure.

FIG. 1 is a conceptual diagram illustrating an example system 2 that maybe used to deliver stimulation therapy to patient 6, which may implementthe techniques of this disclosure. Patient 6 ordinarily, but notnecessarily, will be a human. Therapy system 2 includes implantablestimulator 4, which may also be referred to as an implantable medicaldevice (IMD), that delivers electrical stimulation to patient 6 via oneor more implantable electrodes, such as electrodes 11 on implantablemedical lead 10. The implantable electrodes may be deployed on one ormore implantable medical leads, such as implantable medical lead 10, andin some cases on a can electrode. In other examples an implantablestimulator may be a leadless stimulator including electrodes on anexternal surface of an external housing of the implantable stimulator.Although FIG. 1 shows a fully IMD 4, techniques described in thisdisclosure may be applied to external stimulators having electrodesdeployed via percutaneously implantable leads with a patch electrode orother indifferent electrode attached externally to serve as the can orcase. One or more of the electrodes may be located on a housing 14,i.e., “can” or “case,” of the IMD 4. In addition, in some cases,implantable electrodes may be deployed on a leadless stimulator.

The electrical stimulation may be in the form of controlled current orvoltage pulses or substantially continuous waveforms. Various parametersof the pulses or waveforms may be defined by a stimulation program. Thepulses or waveforms may be delivered substantially continuously or inbursts, segments, or patterns, and may be delivered alone or incombination with pulses or waveforms defined by one or more otherstimulation programs.

In the example illustrated in FIG. 1, IMD 4 is implanted within asubcutaneous pocket in a clavicle region of patient 6. IMD 4 generatesprogrammable electrical stimulation, e.g., a current waveform or currentpulses, and delivers the stimulation via an implantable medical lead 10carrying an array of implantable stimulation electrodes 11. In somecases, multiple implantable leads may be provided. In the example ofFIG. 1, a distal end of lead 10 is bifurcated and includes two leadsegments 12A and 12B (collectively “lead segments 12”). Lead segments12A and 12B each include a set of one or more electrodes forming part ofthe array of electrodes 11. In various examples, lead segments 12A and12B may each carry four, eight, or sixteen electrodes. In FIG. 1, eachlead segment 12A, 12B carries four electrodes, configured as ringelectrodes at different axial positions near the distal ends of the leadsegments. Throughout the remainder of this disclosure, for purposes ofsimplicity, the disclosure may generally refer to electrodes carried on“leads” rather than “lead segments.”

FIG. 1 further depicts a housing, or can, electrode 13. Housingelectrode 13 may be formed integrally with an outer surface ofhermetically-sealed housing 14 of IMD 4 or otherwise coupled to housing14. In one example, housing electrode 13 may be described as an active,non-detachable electrode on the surface of the IMD. In some examples,housing electrode 13 is defined by an uninsulated portion of an outwardfacing portion of housing 14 of IMD 4. Other divisions between insulatedand uninsulated portions of housing 14 may be employed to define two ormore housing electrodes, which may be referred to as case or canelectrodes. In some examples, housing electrode 13 comprisessubstantially all of housing 14, or a portion of the housing 14. Inother examples, electrode 13 may be formed by an electrode on adedicated short lead extending from housing 14. As a furtheralternative, housing electrode 13 could be provided on a proximalportion of one of the leads carrying electrodes 11. The proximal portionmay be closely adjacent to housing 14, e.g., at or near a point at whichlead 10 is coupled to the housing, such as adjacent to a lead connectionheader 8 of the housing. In another example, a patch electrode or otherindifferent electrode may be attached externally to serve as the can orcase.

In some examples, lead 10 may also carry one or more sense electrodes topermit IMD 4 to sense electrical signals from patient 6. Some of thestimulation electrodes may be coupled to function as stimulationelectrodes and sense electrodes on a selective basis. In other examples,IMD 4 may be coupled to one or more leads which may or may not bebifurcated. In such examples, the leads may be coupled to IMD 4 via acommon lead extension or via separate lead extensions.

A proximal end of lead 10 may be both electrically and mechanicallycoupled to header 8 on IMD 4 either directly or indirectly via a leadextension. Conductors in the lead body may electrically connectstimulation electrodes located on lead segments 12 to IMD 4. Lead 10traverses from the implant site of IMD 4 along the neck of patient 6 tocranium 18 of patient 6 to access brain 16. Lead segments 12A and 12Bmay be implanted within the right and left hemispheres, respectively, inorder to deliver electrical stimulation to one more regions of brain 16,which may be selected based on the patient condition or disorder.

IMD 4 may deliver, for example, deep brain stimulation (DBS) or corticalstimulation (CS) therapy to patient 6 via the electrodes carried by,i.e., located on, lead segments 12 to treat any of a variety ofneurological disorders or diseases. Example neurological disorders mayinclude depression, dementia, obsessive-compulsive disorder and movementdisorders, such as Parkinson's disease, spasticity, epilepsy, anddystonia. DBS also may be useful for treating other patient conditions,such as migraines and obesity. However, the disclosure is not limited tothe configuration of lead 10 shown in FIG. 1, or to the delivery of DBSor CS therapy.

Lead segments 12A, 12B are implanted within a desired location of brain16 through respective holes in cranium 18. Lead segments 12A, 12B may beplaced at any location within brain 16 such that the electrodes locatedon lead segments 12A, 12B are capable of providing electricalstimulation to targeted tissue during treatment.

The electrodes of lead segments 12A, 12B are shown as ring electrodes.Ring electrodes are commonly used in DBS applications because they maybe relatively simple to program and are capable of delivering anelectrical field to any tissue adjacent to lead segments 12A, 12B. Inother implementations, the electrodes of lead segments 12A, 12B may havedifferent configurations. For example, the electrodes of lead segments12A, 12B may have a complex electrode array geometry that is capable ofproducing shaped electrical fields.

Therapy system 2 also includes a programmer 20, which may be a clinicianor patient programmer. Programmer 20 may be a handheld computing devicethat permits a user to program stimulation therapy for patient 6 via auser interface, e.g., using input keys and a display. For example, usingprogrammer 20, the clinician may specify stimulation parameters, i.e.,create programs, for use in delivery of stimulation therapy. Programmer20 may support telemetry (e.g., radio frequency (RF) telemetry) with IMD4 to download programs and, optionally, upload operational orphysiological data stored by IMD 4. In this manner, a user, such as thepatient or the clinician, may periodically interrogate IMD 4 to evaluateefficacy and, if necessary, modify the programs or create new programs.In some examples, clinician programmer transmits programs to patientprogrammer in addition to or instead of IMD 4. A clinician programmermay be used more extensively in programming and downloading therapy, andmay have more capabilities, such as, for example, the ability to changemore therapy parameters than a patient programmer.

A patient programmer may be a handheld computing device, and may includea display and input keys to allow patient 6 to interact with patientprogrammer and IMD 4. In this manner, patient programmer providespatient 6 with a user interface for control of the stimulation therapydelivered by IMD 4. For example, patient 6 may use patient programmer tostart, stop or adjust electrical stimulation therapy. In particular,patient programmer may permit patient 6 to adjust stimulation parametersof a program such as electrode combination, electrode polarities,duration, current or voltage amplitude, pulse width and pulse rate.Patient 6 may also select a program, e.g., from among a plurality ofstored programs, as the present program (or one of a plurality ofprograms in a program group) to control delivery of stimulation by IMD4.

IMD 4 and programmer 20 may communicate via wireless communication, asshown in FIG. 1. For example, clinician programmer and patientprogrammer may communicate with each other using any of a variety oflocal wireless communication techniques, such as RF communicationaccording to the 802.11 or Bluetooth specification sets, infraredcommunication, e.g., according to the IrDA standard, or other standardor proprietary telemetry protocols. Programmer 20 may include atransceiver to facilitate bi-directional communication with IMD 4.

In some examples, as described in greater detail with respect to FIGS. 3and 4 below, programmer 20 may be used to recharge a power source of IMD4 using the techniques of this disclosure. That is, for example,programmer 20 may include one or more leads, inductive coils, or othercomponents for recharging a power source of IMD 4, e.g., viatranscutaneous inductive coupling or other electrical or electromagneticcoupling. Alternatively, a charging device (not shown) having morelimited (or even no) programming capabilities may be used to rechargethe power source of IMD 4. Programmer 20 may also receive a number ofmeasurements from the IMD 4 that are used to control a charging session.According to the techniques of this disclosure, IMD 4 may be chargedduring a top-off period that is based on a capacity of the power sourceof IMD 4, as well as a state of charge of the power source. Additionallyor alternatively, according to the techniques of this disclosure, IMD 4may be charged during a top-off period that includes a chargingtermination parameter that is determined based on a duration of aprevious charging session and/or an amount of charge that wasadministered to the power source during the previous charging session.

FIG. 2 is a conceptual diagram illustrating system 30 that deliversstimulation therapy to spinal cord 38 of patient 36 via implantablestimulator 34 (which may be referred to as IMD 34), with may implementthe techniques of this disclosure. Other electrical stimulation systemsmay be configured to deliver electrical stimulation to gastrointestinalorgans, pelvic nerves or muscle, peripheral nerves, or other stimulationsites. In the example of FIG. 2, system 30 delivers stimulation therapyfrom IMD 34 to spinal cord 38 via one or more electrodes (not shown)carried by, i.e., located on, implantable medical leads 32A and 32B(collectively “leads 32”) as well as the housing of IMD 34, e.g.,housing electrode 37.

System 30 and, more particularly, IMD 34 may operate in a manner similarto implantable stimulator 4 (FIG. 1). That is, in a current-basedexample, IMD 34 delivers controlled current stimulation pulses orwaveforms to patient 36 via one or more regulated stimulationelectrodes. Alternatively, IMD 34 may be configured to deliver constantvoltage pulses. As mentioned above, in some examples, one of theelectrodes may be unregulated.

In the example of FIG. 2, the distal ends of leads 32 carry electrodesthat are placed adjacent to the target tissue of spinal cord 38. Theproximal ends of leads 32 may be both electrically and mechanicallycoupled to IMD 34 either directly or indirectly via a lead extension andheader. Alternatively, in some examples, leads 32 may be implanted andcoupled to an external stimulator, e.g., through a percutaneous port. Inadditional example implementations, IMD 34 may be a leadless stimulatorwith one or more arrays of electrodes arranged on a housing of IMD 34rather than leads that extend from the housing. Implantable leads 32 mayhave ring electrodes for purposes of illustration. However, other typesof electrodes may be used.

IMD 34 may be implanted in patient 36 at a location minimally noticeableto the patient. For SCS, IMD 34 may be located in the lower abdomen,lower back, or other location to secure IMD 34. Leads 32 are tunneledfrom IMD 34 through tissue to reach the target tissue adjacent to spinalcord 38 for stimulation delivery. At the distal ends of leads 32 are oneor more electrodes (not shown) that transfer the stimulation pulses fromthe lead to the tissue substantially simultaneously with stimulationpulses. Some of the electrodes may be electrode pads on a paddle lead,circular (i.e., ring), electrodes surrounding the body of leads 32,conformable electrodes, cuff electrodes, segmented electrodes, or anyother type of electrodes capable of forming unipolar, bipolar ormulti-polar electrode configurations. In an implantable stimulator, suchas, for example, an implantable stimulator of FIG. 1 or FIG. 2, thestimulation pulses may be delivered using various electrode arrangementssuch as unipolar arrangements, bipolar arrangements or multipolararrangements.

IMD 34 delivers stimulation to spinal cord 38 to reduce the amount ofpain perceived by patient 36. As mentioned above, however, thestimulator may be used with a variety of different therapies, such asperipheral nerve stimulation (PNS), peripheral nerve field stimulation(PNFS), deep brain stimulation (DBS), cortical stimulation (CS), pelvicfloor stimulation, peripheral nerve stimulation, gastric stimulation,and the like. The stimulation delivered by IMD 34 may take the form ofstimulation pulses or continuous stimulation waveforms, and may becharacterized by controlled current or voltage levels, as well asprogrammed pulse widths and pulse rates in the case of stimulationcurrent pulses. Stimulation may be delivered via selected combinationsof electrodes located on one or both of leads 32 and on the housing.Stimulation of spinal cord 38 may, for example, prevent pain signalsfrom traveling through the spinal cord and to the brain of the patient.Patient 36 perceives the interruption of pain signals as a reduction inpain and, therefore, efficacious therapy.

With reference to FIG. 2, a user, such as a clinician or patient 36, mayinteract with a user interface of external programmer 40 to program IMD34. Programming of IMD 34 may refer generally to the generation andtransfer of commands, programs, or other information to control theoperation of the stimulator. For example, programmer 40 may transmitprograms, parameter adjustments, program selections, or otherinformation to control the operation of IMD 34, e.g., by wirelesstelemetry.

As noted above, in some examples, programmer 40 may be used to rechargea power source of IMD 34 using the techniques of this disclosure. Thatis, for example, programmer 40 may include one or more leads, inductivecoils, or other components for recharging a power source of IMD 34,e.g., via transcutaneous inductive coupling or other electrical orelectromagnetic coupling. Alternatively, a charging device (not shown)having more limited (or even no) programming capabilities may be used torecharge the power source of IMD 34. Programmer 40 may also receive anumber of measurements from the IMD 34 that are used to control acharging session. According to the techniques of this disclosure, IMD 34may be charged during a top-off period that is based on a capacity ofthe power source of IMD 34, as well as a state of charge of the powersource. Additionally or alternatively, according to the techniques ofthis disclosure, IMD 34 may be charged during a top-off period thatincludes a charging termination parameter that is determined based on aduration of a previous charging session and/or an amount of charge thatwas administered to the power source during the previous chargingsession.

Although the disclosure generally refers to implantable electricalstimulators for purposes of illustration, techniques described in thisdisclosure may be also used with other types of implantable medicaldevices, including implantable fluid delivery devices, such as insulinpumps, intra-thecal drug delivery pumps, or other devices that delivermedication or other fluids via one or more fluid delivery elements suchas catheters. Such devices may provide fluid delivery therapy forchronic pain, diabetes, or any of a variety of other disorders. In eachcase, the device may include one or more therapy delivery elements suchas one or more catheters implanted within a therapy region. In somecases, a pump may be fully implantable or may be an external devicecoupled to one or more percutaneously implanted catheters that extendinto a therapy region. In some examples, the techniques of thisdisclosure may be also used with external neural stimulators, such as,for example, those used for “trialing” therapies and/or devices with apatient prior to implantation. Accordingly, description of implantablestimulators is provided for purposes of illustration and should not beconsidered limiting of the techniques as broadly described in thisdisclosure.

FIG. 3 is a block diagram illustrating various components of an exampleimplantable stimulator, such as IMD 34 shown in FIG. 2, which mayimplement the techniques of this disclosure. Although the componentsshown in FIG. 3 are described in reference to IMD 34, such componentsmay also be included within implantable stimulator 4 and used withinsystem 2 (FIG. 1), or within another implantable stimulator. In theexample of FIG. 3, IMD 34 includes electrodes 48A-48Q (“electrodes 48”),stimulation generator 49, processor 50, memory 52, telemetry module 53,antenna 54, power source 56, coulomb counter 62, charging module 58, andcharging coil 60. It should be understood that, in other examples, IMD34 may include more or fewer components than those shown in FIG. 3. Forexample, as described in greater detail below, power source 56 may beassociated with one or more inductive coils not shown in FIG. 3 forpurposes of clarity.

IMD 34 is also shown in FIG. 3 coupled to electrodes 48A-Q (collectively“electrodes 48”). Electrodes 48 may be implantable and may be deployedon one or more implantable leads. With respect to FIG. 1, lead segments12A and 12B may carry electrodes 48A-H and electrodes 48I-P,respectively. In some cases, one or more additional electrodes may belocated on or within the housing of IMD 34, e.g., to provide a common orground electrode or a housing anode. With respect to FIG. 3, leads 32Aand 32B may carry electrodes 48A-H and electrodes 48I-P, respectively.In the examples of FIGS. 1 and 2, a lead or lead segment carries eightelectrodes to provide an 2×8 electrode configuration (two leads with 8electrodes each), providing a total of sixteen different electrodes. Theleads may be detachable from a housing associated with IMD 34, or befixed to such a housing.

In other examples, different electrode configurations comprising asingle lead, two leads, three leads, or more may be provided. Inaddition, electrode counts on leads may vary and may be the same ordifferent from a lead to lead. Examples of other configurations includeone lead with eight electrodes (1×8), one lead with 12 electrodes(1×12), one lead with 16 electrodes (1×16), two leads with fourelectrodes each (2×4), three leads with four electrodes each (3×4),three leads with eight electrodes each (3×8), three leads with four,eight, and four electrodes, respectively (4-8-4), two leads with 12 or16 electrodes (2×12, 2×16), or other configurations. Differentelectrodes are selected to form electrode combinations. Polarities areassigned to the selected electrodes to form electrode configurations.

Electrode 48Q may represent one or more electrodes that may be carriedon a housing, i.e., can, of IMD 34. Electrode 48Q may be configured as aregulated or unregulated electrode for use in an electrode configurationwith selected regulated and/or unregulated electrodes among electrodes48A-48P, which may be located on a lead body of one or more leads, asdescribed above. Electrode 48Q may be formed together on a housing thatcarries the electrode and houses the components of IMD 34, such asstimulation generator 49, processor 50, memory 52, telemetry module 53,and power source 56.

In addition, electrode 48Q may be configured for use as an anode tosource current substantially simultaneously with current sourced by oneor more other electrodes 48A-48P to form a unipolar or omnipolararrangement. By way of specific example, electrodes 48A, 48B, andelectrode 48Q each could be configured for use as anodes. Electrodes48A, 48B could deliver electrical stimulation current substantiallysimultaneously with the electrical stimulation current delivered viaelectrode 48Q. In this illustration, one or more cathodes could beformed with other electrodes (e.g., any of electrodes 48C-48P) on theleads to sink current sourced by anodes 48A, 48B and 48Q. Any of avariety of electrode arrangements such as unipolar, bipolar, multipolar,or omnipolar arrangements may be used to deliver stimulation.Accordingly, discussion of particular arrangements is provided forpurposes of illustration which should not be considered limiting of thetechniques broadly described in this disclosure.

Stimulation generator 49 is electrically coupled to electrodes 48A-P viaconductors of the respective lead, such as lead 12 in FIG. 1 or leads 32in FIG. 2, in implementations in which electrodes 48A-P are carried by,or located on, leads. Stimulation generator 49 may be electricallycoupled to one or more housing (“can”) electrodes 48Q via an electricalconductor disposed within the housing of IMD 34. Stimulation generator49 may include stimulation generation circuitry to generate stimulationpulses or waveforms and circuitry for switching stimulation acrossdifferent electrode combinations. For example, stimulation generator 49may produce an electrical stimulation signal in accordance with aprogram based on control signals from a processor, such as processor 50.

Processor 50 may include one or more microprocessors, digital signalprocessors (DSPs), application-specific integrated circuits (ASICs),field-programmable gate arrays (FPGAs), or other digital logiccircuitry. Processor 50 may control operation of IMD 34, e.g., controlsstimulation generator 49 to deliver stimulation therapy according to aselected program or group of programs retrieved from a memory, such asmemory 52 described below. For example, processor 50 may controlstimulation generator 49 to deliver electrical signals, e.g., asstimulation pulses or continuous waveforms, with current amplitudes,pulse widths (if applicable), and rates specified by one or morestimulation programs. Processor 50 may control stimulation generator 49based on parameters specified by programs downloaded from an externalprogrammer such as, for example, programmer 20 (FIG. 1) or programmer 40(FIG. 2). An external programmer, such as a clinician or patientprogrammer, may also specify that processor 50 should select one or moreprograms that have been downloaded to the implantable stimulator.

Upon selection of a particular program, processor 50 may controlstimulation generator 49 to deliver stimulation according to theprogram. In some examples, multiple programs may be selected forseparate program slots. Each program may specify a set of stimulationparameters, such as amplitude, pulse width and pulse rate, ifapplicable. For a continuous waveform, parameters may include amplitudeand frequency. In addition, each program may specify a particularelectrode combination for delivery of stimulation, and an electrodeconfiguration in terms of the polarities and regulated/unregulatedstatus of the electrodes. The electrode combination may specifyparticular electrodes in a single array or multiple arrays, and on asingle lead or among multiple leads. According to aspects of thisdisclosure, processor 50 may also control charging of power source 56.For example, processor 50 may control charging module 58 when chargingpower source 56.

Memory 52 may store instructions for execution by processor 50,stimulation therapy data, sensor data, and/or other informationregarding therapy for patient 6. Memory 52 may comprise one or morecomputer-readable storage media. Examples of memory 52 include, but arenot limited to, a random access memory (RAM), a read only memory (ROM),an electrically erasable programmable read-only memory (EEPROM), amagnetic storage device, flash memory, or any other medium that can beused to store desired program code in the form of instructions or datastructures and that can be accessed by a computer or a processor. Memory52 may, in some examples, be considered as a non-transitory storagemedium. In certain examples, a non-transitory storage medium may storedata that can, over time, change (e.g., in RAM).

As noted above, in some examples, memory 52 may store programinstructions that, when executed by processor 50, cause the processor toperform various functions ascribed to processor 50 and IMD 34 in thisdisclosure. Memory 52 may also store a patient profile and informationregarding therapy that the patient 6 had previously received. Storingsuch information may be useful for subsequent treatments such that, forexample, a clinician may retrieve the stored information to determinethe therapy applied to the patient during his/her last visit. Theinformation may be modified and updated by a user of a programmer.

Telemetry module 53 may include a radio frequency (RF) transceiver tofacilitate bi-directional communication between IMD 34 and a programmer,such as programmer 40 (FIG. 2). Telemetry module 53 may include anantenna 54 that may take on a variety of forms. For example, antenna 54may be formed by a conductive coil or wire embedded in a housingassociated with IMD 34. Alternatively, antenna 54 may be mounted on acircuit board carrying other components of IMD 34 or take the form of acircuit trace on the circuit board. In this way, telemetry module 53 mayfacilitate communication with programmer 40 to receive, for example, newprograms or adjustments to programs.

Power source 56 may be any unit that provides power to the components ofIMD 34 by discharging charge that is stored within power source 56. Insome examples, power source 56 may be a rechargeable battery and may becoupled to power circuitry. That is, power source 56 may be one or morerechargeable batteries that are tied together in parallel or in seriesto form a single power source. In another example, power source 56 maycomprise one or more single use batteries (e.g., non-rechargeable), oneor more capacitors, and/or supercapacitors. In addition, while shown inFIG. 3 as a single power source 56, IMD 34 may include multipledifferent power sources 56. Accordingly, in examples in which IMD 34includes multiple different power sources, aspects of this disclosuremay be extendable to each power source.

As noted above, power source 56 may include one or more a rechargeablebatteries or a non-rechargeable battery or batteries, e.g., one or moreprimary cell batteries. Examples of power source 56 include, but are notlimited to, lead acid batteries, nickel cadmium (NiCad) batteries,nickel metal hydride (NiMH) batteries, lithium ion (Li-ion) batteries,and lithium ion polymer (Li-ion polymer) batteries.

Power source 56 may provide power to one, some, or all of the variouscomponents of IMD 34. Accordingly, power source 56 may be discharged dueto the power consumed by the various components of IMD 34 (e.g., such asstimulation generator 49, processor 50, telemetry module 53, or anyother component of IMD 34). Due to the discharging, power source 56 mayneed to be recharged or replaced periodically to ensure that powersource 56 does not fully drain. As described in greater detail below,aspects of this disclosure relate to recharging power source 56. Morespecifically, certain aspects of this disclosure may relate torecharging power source 56 during a top-off period, typically toward theend of a charging session.

Charging module 58 may facilitate charging of power source 56. Forexample, power source 56 may be rechargeable via electrical orelectromagnetic coupling, such as inductive coupling, or ultrasonicenergy transmission, and may include an appropriate circuit forrecovering transcutaneously received energy. In the example shown inFIG. 3, charging module 58 includes charging coil 60 for inductiveenergy transfer. In some examples, charging module 58 may also include arectifier circuit, as well as a rechargeable circuit and a powergeneration circuit to produce the operating power. Recharging may beaccomplished through proximal inductive interaction between an externalcharger (e.g., included in programmer 40 or a charging device) andcharging coil 60. In some examples, power requirements may be smallenough to allow IMD 34 to utilize patient motion at least in part andimplement a kinetic energy-scavenging device to trickle charge powersource 56.

In general, coulomb counter 62 may monitor the charge delivered by powersource 56 and/or the charge received by power source 56 (e.g., uponrecharging). For example, coulomb counter 62 may be initialized to zero.In some examples, for every unit of charge that is received by powersource 56, coulomb counter 62 may increment a counter by one. Forexample, during charging of power source 56, coulomb counter 62 maycount each unit of charge (e.g., in coulombs) delivered to power source56. Alternatively, for every unit of charge delivered by power source56, coulomb counter 62 may decrement a counter by one. That is, forexample, when providing a therapy via stimulation generator 49, coulombcounter may count each unit of charge (e.g., in coulombs) delivered bypower source 56. In some examples, to count the amount of chargedelivered and/or received by power source 56, coulomb counter 62 mayintegrate the amount of current delivered by power source 56 over time.

Accordingly, in some examples, coulomb counter 62 may be positioned atthe output of power source 56 to accurately reflect the amount ofcurrent or charge being delivered to stimulation generator 49 duringtherapy and/or may be positioned at the input of power source to reflectthe amount of current or charge being received by/administered to powersource 56 during a charging session. In some examples, coulomb counter62 may comprise two coulomb counters, with one determining the amount ofcurrent or charge delivered by power source 56 and the other determiningthe amount of current or charge delivered to power source 56 so that theoverall charge stored by power source may be determined. If coulombcounter 62 is configured to measure coulombs, the actual amount ofcharge transferred to the electrodes may be determined. If current ismeasured, the integral of current over the time that stimulation isdelivered will yield the number of milliamp-hours. In either event,using the relationship that one coulomb equals approximately 0.00027778amp-hours, the charge delivered and/or received by power source 56 maybe determined.

According to the techniques of this disclosure, IMD 34 may be chargedduring a top-off period that is based on a capacity of power source 56,as well as a state of charge of power source 56. Additionally oralternatively, according to the techniques of this disclosure, IMD 34may be charged during a top-off period that includes a chargingtermination parameter that is determined based on a duration of aprevious charging session and/or an amount of charge that wasadministered to power source 56 during the previous charging session.

For example, a charging top-off period typically occurs during the endof a charging session (e.g., as power source 56 approaches full charge).The top-off period may help to charge power source 56 to full capacity,while also reducing the risk of raising the voltage of power source 56beyond a desirable level. That is, during charging, an impedance ofpower source 56 may increase as power source 56 reaches full charge. Toavoid a corresponding rise in voltage, which may cause power source 56to swell and/or become unstable, a charging current may be reducedduring a top-off period. In this way, power source 56 may be fullycharged without increasing the voltage of power source 56 in apotentially undesirable way.

According to aspects of this disclosure, IMD 34 may estimate thecapacity of power source 56 based on an amount of charge applied topower source 56 during a previous charging session. For example, coulombcounter 62 may track an amount of charge that is expended by IMD 34, aswell as an amount of charge that is stored by power source 56 during acharging session, assuming power source 56 is fully charged during eachcharging session. Accordingly, IMD 34 may estimate a new capacity forpower source after each charging session, based on the amount of chargethat was expended versus the amount of charge that was administeredduring the charging session. In this way, IMD 34 may adjust the capacityof power source 56 as power source 56 degrades and is able to storerelatively less charge.

IMD 34 may determine the state of charge of power source 56 relative tothe capacity of power source 56. In some examples, IMD 34 may usecoulomb counter 62 to determine the state of charge. That is, IMD 34 maydetermine the number of coulombs (or current) being received by powersource 56 from charging module 58 relative to the capacity of powersource 56. According to aspects of this disclosure, IMD 34 may reducethe charging current that is applied to power source 56 from chargingmodule 58 in response to the state of charge of power source 56approaching the capacity of power source 56.

As noted above, according to the techniques of this disclosure, IMD 34may additionally or alternatively be charged during a top-off periodthat includes a charging termination parameter that is determined basedon a duration of a previous charging session and/or an amount of chargethat was administered to power source 56 during the previous chargingsession. Examples of charging termination parameters may include aminimum charging current cut off, a power level cut off, or otherparameters that indicate when power source 56 has reached full charge(e.g., impedance of power source 56, and the like).

A minimum charging current cut off may be associated with the smallestcurrent at which IMD 34 is allowed to charge power source 56. That is,for example, it may not be beneficial to charge power source 56 using acharging current that is lower than the minimum charging current, aspower source 56 may not be effectively charged, the charging session maybe longer in duration than desired, and the like. A power level cut offmay be associated with a minimum power with which power source 56 may becharged, which may occur when power source 56 has been fully rechargedor nearly fully recharged. For example, as described below, as powersource 56 approaches full charge, the power used to charge the powersource 56 may be reduced (e.g., during a top-off period). The power cutoff value may be a minimum power level that is used for charging powersource 56. The power cut off value may be set such that the power levelcut off occurs as power source 56 reaches full charge. In someinstances, the power level cut off value may be measured by programmer40 or anther charging device responsible for applying a charge to powersource 56. That is, the power level cut off value may be a minimumoutput power of the charging device.

According to aspects of this disclosure, IMD 34 may initially determinea duration of a previous charging session, as well as, in some examples,an amount of charge that was administered to power source 56 by chargingmodule 58 during the previous charging session. The duration may bemeasured by timing the previous charging session. The amount of chargeadministered to power source 56 by charging module 58 may be determined,for example, using coulomb counter 62.

IMD 34 may then determine a new minimum charging current cut off (e.g.,for the next charging session) based on the duration of the previouscharging session and/or the amount of charge administered during theprevious charging session. For example, according to aspects of thisdisclosure, if the amount of charge delivered during the previouscharging session does not charge power source 56 to full or nearly fullcapacity, IMD 34 may adjust a charging termination parameter, such aslower the minimum charging current cut off. Lowering the minimumcharging current cut off may increase the duration of the top-off periodand allow more charge to be administered to power source 56, i.e., bypermitting the current to be delivered for a longer period of time.

Additionally or alternatively, according to aspects of this disclosure,IMD 34 may compare the duration of the previous charging session to apredefined target duration. If the duration of the previous chargingsession was shorter than the target duration (e.g., power source 56completed charging in less time than the target duration), IMD 34 mayadjust the charging termination parameter allow power source 56 to becharged for a longer duration (e.g., charged using a lower minimumcharging current cut off). Alternatively, if the duration of theprevious charging session was longer than the target duration, IMD 34may adjust the charging termination parameter to shorten the chargingduration.

While IMD 34 shown in FIG. 3 is described as carrying out certaintechniques of this disclosure, e.g., charging IMD 34 during a top-offperiod, it should be understood that the techniques of this disclosuremay be performed by a single device or by multiple devices. That is, insome examples as described above, IMD 34 may be responsible forcontrolling charging parameters (e.g., charging based on a state ofcharge and a capacity of power source 56, altering a minimum chargingcurrent cut off, and the like). In other examples, IMD 34 may providecertain data (e.g., charging current, state of charge, voltage, and thelike) to programmer 40 (or another charging device), thereby allowingprogrammer 40 to control charging parameters. In still other examples,IMD 34 and programmer 40 may each be responsible for controllingcharging functions.

FIG. 4 is a functional block diagram illustrating various components ofan external programmer for an implantable stimulator, such as IMD 4(FIG. 1) and/or IMD 34 (FIG. 2). Although the components shown in FIG. 4are described in reference to external programmer 40 shown in FIG. 2, itshould be understood that the components may also be included within aprogrammer 20 (FIG. 1), or another clinician programmer and/or patientprogrammer.

As shown in FIG. 4, external programmer 40 includes user interface 63,processor 64, memory 65, telemetry module 66, antenna 68, power source70, and charging module 72 having charging coil 74. The components shownin FIG. 4 are provided for purposes of example only, and other externalprogrammers may include more or fewer components than those shown inFIG. 4.

A clinician or patient 6 interacts with user interface 63 in order to,for example, manually select, change or modify programs, e.g., byadjusting voltage or current amplitude, adjusting pulse rate, adjustingpulse width, or selecting different electrode combinations, polarities,or configurations, and may provide efficacy feedback or view stimulationdata. User interface 63 may include a screen and one or more input hardand/or soft key buttons that allow external programmer 20 to receiveinput from a user. The screen may be a liquid crystal display (LCD),plasma display, dot matrix display, or touch screen. The input buttonsmay include a touch pad, increase and decrease buttons, emergency shutoff button, and other input media needed to control the stimulationtherapy.

The user interface 63 may also display information associated with acharging session, in examples in which programmer 20 is used to charge arechargeable power source of an implantable stimulator, such as powersource 56. For example, user interface 63 may display informationindicating a total remaining charge of power source 56. User interface63 may also display a timer that tracks how long a particular chargingsession has lasted. In some examples, user interface 63 may indicate acharging current used to charge power source 56 (e.g., in milliamps(mA)). User interface 63 may also indicate a state of charge (e.g., inmilliamp-hours (mAh) or as a percentage of total charge), as well aswhen the power source 56 has been fully charged.

In general, processor 64 controls user interface 63, stores andretrieves data to and from a memory (e.g., such as memory 65 describedbelow), and controls an exchange of data with IMD 34 via telemetrymodule 66 and antenna 68. Processor 64 may take the form of one or moremicroprocessors, controllers, DSPs, ASICS, FPGAs, or equivalent discreteor integrated logic circuitry. The functions attributed to processor 64herein may be embodied as software, firmware, hardware or anycombination thereof

Memory 65 may store instructions that cause processor 64 to providevarious aspects of the functionality ascribed to external programmer 20herein. Memory 65 may include any fixed or removable magnetic, optical,or electrical media, such as RAM, ROM, CD-ROM, magnetic disks, EEPROM,or the like. Memory 65 may also include a removable memory portion thatmay be used to provide memory updates or increases in memory capacities.A removable memory may also allow patient data to be easily transferredto another computing device, or to be removed before programmer 20 isused to program therapy for another patient. Memory 65 may also storeinformation that controls operation of IMD 34, such as therapy deliveryvalues and/or charging parameters.

Telemetry module 66 allows the transfer of data to and from IMD 34.Telemetry module 66 may communicate automatically with IMD 34 at ascheduled time or when the telemetry module 66 detects the proximity ofthe stimulator. Alternatively, telemetry module 66 may communicate withIMD 34 when signaled by a user through user interface 63. To support RFcommunication, telemetry module 66 may include appropriate electroniccomponents, such as amplifiers, filters, mixers, encoders, decoders, andthe like. As described with respect to FIG. 3 above, telemetry module 66may include an antenna 68 that may take on a variety of forms. Forexample, antenna 68 may be formed by a conductive coil or wire embeddedin a housing associated with programmer 20. Alternatively, antenna 68may be mounted on a circuit board carrying other components ofprogrammer 20 or take the form of a circuit trace on the circuit board.In accordance with this disclosure, programmer 40 may communicate withIMD 34, via telemetry module 66 to retrieve information during acharging session.

Power source 70 delivers operating power to the components of programmer40. Power source 70 may be a rechargeable battery, such as a lithium ionor nickel metal hydride battery. Other rechargeable or conventionalbatteries may also be used. In some cases, external programmer 20 may beused when coupled to an alternating current (AC) outlet, i.e., AC linepower, either directly or via an AC/DC adapter. Power source 70 mayinclude circuitry to monitor energy remaining within a battery in theprogrammer 40. In this manner, user interface 63 may provide a currentbattery level indicator or low battery level indicator when the batteryof programmer 40 needs to be replaced or recharged. In some cases, powersource 70 may be capable of estimating the remaining time of operationusing the current battery.

Charging module 72 may be used to recharge a rechargeable battery of animplantable stimulator, such as IMD 34. Charging module 72 may receivecharge from one or more power sources, such as a wired power connectionor one or more batteries (e.g., rechargeable or non-rechargeable). Inthe example shown in FIG. 4, charging module 72 includes charging coil74 for inductive energy transfer. In some examples, charging coil 74 maybe an external coil that may be placed on the surface of the skin ofpatient 36 in proximity to a coil of an implanted device, such as IMD34, to perform inductive energy transfer. Thus, in some examples,charging module 72 may be configured to transcutaneously charge a powersource of an implantable stimulator, such as power source 56, e.g.,without a direct physical connection via leads or other wires. In someexamples, charging module 72 may be integrated with telemetry module 66,but is shown separately in FIG. 4 for purposes explanation.

In some examples, as described above with respect to FIG. 3, IMD 34 maybe responsible for controlling charging. In other examples, however,programmer 40 or some other charging device may be responsible forcarrying out charging. For example, processor 64 may be responsible formonitoring charging parameters and controlling charging module 72. Thatis, programmer 40 may receive certain data from IMD 34 and may control acharging session based on the received data. In an example, IMD 34 maysend data such as charging current, state of charge, voltage, capacity,and the like to programmer 40. Upon receiving the feedback data,programmer 40 may adjust output power at charging module 72 and chargingcoil 74.

Additionally or alternatively, in some examples, IMD 34 and programmer40 may collaboratively control a charging session. For example, IMD 34may monitor a voltage of power source 56 during charging, and shuntexcess current (e.g., divert to a resistive load) if the voltageincreases beyond a predetermined limit. In this example, IMD 34 mayreport the voltage limit condition to programmer 40, which may use thereceived data to alter the charging parameters (e.g., reduce outputpower at charging module 72 and charging coil 74). After adjusting thecharging parameters such that IMD 34 is no longer shunting chargingcurrent, programmer 40 may determine whether to continue charging.

Accordingly, in some examples, programmer 40 may implement thetechniques of this disclosure. For example, programmer 40 may implementa top-off period that is based on a capacity of a power source and astate of charge of the power source of an implantable stimulator, suchas IMD 34. Additionally or alternatively, programmer 40 may implement atop-off period that includes a charging termination parameter that isdetermined based on a duration of a previous charging session and/or anamount of charge that was administered to the power source during theprevious charging session.

FIG. 5 is a graph illustrating an example charging session having atop-off period. For example, FIG. 5 generally illustrates an examplecharging session in which a voltage limit is used during a top-offperiod to control a charging current applied to a power source (e.g., abattery). For example, as noted above, a charging top-off periodtypically occurs during the end of a charging session, and may help tocharge the power source to full capacity without increasing the voltageof the power source beyond a desirable level. That is, during charging,an impedance of the power source may increase as the power sourcereaches full charge. To avoid a corresponding rise in voltage, acharging current may be reduced during a top-off period. In the exampleshown in FIG. 5, the top-off may be triggered by the voltage reachingthe voltage limit. The top-off period then continues until the powersource has reached full charge.

Although generally described as being performed by components of IMD 34(FIGS. 2, 3) and programmer 40 (FIGS. 2, 4) for purposes of explanation,it should be understood that other implantable stimulators andprogrammers, such as implantable stimulator 4 and programmer 20 (FIG.12), or a variety of other devices may also be configured to perform thecharging session shown in FIG. 5. Moreover, in other examples, IMD 34and/or programmer 40 may perform different functions than thosedescribed with respect to FIG. 5 (e.g., IMD 34 may perform functionsattributed to programmer 40, and vice versa).

The example graph shown in FIG. 5 tracks a charging current 80, avoltage 82, and a state of charge 84 throughout a charging session ofpower source 56. That is, for example, charging current 80 correspondsto a current induced in charging coil 60 of charging module 58 andapplied to power source 56. In the example shown in FIG. 5, IMD 34measures charging current 80, which is represented in milliamps (mA).Voltage 82 corresponds to a voltage measured at power source 56, whichmay be measured in volts (V). In addition, state of charge 84corresponds to a state of charge of power source 56, which may bemeasured in milliamp-hours (mAh). In other examples, state of charge 84may also be represented as a percentage of the capacity of power source56 (e.g., 50% of full charge). In some examples, IMD 34 may relay thisdata to programmer 40 (or another charging device) during a chargingsession.

As shown in FIG. 5, programmer 40 applies a charging current 80 to powersource 56 during an initial charging period 86, e.g., via inductivecoils and suitable charging circuitry within programmer 40 and withinIMD 34. During the initial charging period 86, programmer 40 may apply amaximum charging current (e.g., as measured at power source 56) to powersource 56 in order to charge power source 56 as quickly as possiblewithout damaging power source 56. In the example shown in FIG. 5,programmer 40 applies a current that, upon inductive transfer, induces amaximum charging current of approximately 80 milliamps (mA), as appliedto power source 56, during the initial charging period 86. In otherexamples, the maximum charging current may vary depending on, forexample, the capabilities of power source 56, the capabilities ofprogrammer 40, or other factors.

In the example shown in FIG. 5, IMD 34 initiates a top-off period 88upon the voltage 82 of power source 56 reaching a voltage limit (“maxvoltage”). For example, IMD 34 may indicate to programmer 40 that IMD 34has reached the voltage limit. The voltage limit may be reached, forexample, due to rising impedance in power source 56 as power source 56is charged. That is, if a constant maximum charging current is appliedto power source 56, as impedance in power source 56 increases voltagealso increases.

When the voltage limit is reached, programmer 40 may begin to top-offpower source 56 of IMD 40 using a lower charging current (e.g., viacharging module 72 and charging coil 74) in an attempt to maximize thecharge delivered to power source 56. The voltage limit that triggerstop-off may be a predefined voltage level that is intended to protectpower source 56 from swelling or otherwise becoming unstable duringcharging. In the example shown in FIG. 5, the voltage 82 reaches thevoltage limit at a state of charge 84 of approximately 80 mAh.

During the top-off period 88, IMD 34 shunts the charging current 80 suchthat less than the maximum charging current is used to charge powersource 56. That is, power source 56 shunts the charging current 80(e.g., prevents charging current from increasing) as power source 56reaches full charge in order to maintain the voltage 82 of power source56 at the voltage limit. In some examples, IMD 34 may shunt the chargingcurrent, for example, by diverting the charging current to a resistiveload. In other examples, IMD 34 may increase the resistive path betweenpower source 56 and charging coil 60 in an effort to divide the voltagebetween internal impedance of power source 56 and the charging circuitryof IMD 34. In other examples, IMD 34 may communicate with programmer 40to lower the charging current that is applied by charging coil 74 ofcharging module 72 to charging coil 60 of charging module 58.

Programmer 40 may stop charging power source 56 upon determining thatpower source 56 has reached full charge. That is, IMD 34 may relay toprogrammer 40 that power source 56 has reached full charge, andprogrammer 40 may end the charging session. In some examples, asdescribed in greater detail below, programmer 40 may determine thatpower source 56 has reached full charge upon reducing the chargingcurrent to a minimum allowed charging current. That is, programmer maystop charging power source 56 when the charging current declines (duringtop-off) to a minimum charging current. In other examples, programmer 40may determine that power source 56 has reached full charge differently(e.g., using coulomb counter 62).

IMD 34 may initiate top-off period 88 and reduce charging current 80based on reaching the voltage limit alone. However, this may not accountfor variation and/or changing characteristics of power source 56 (e.g.,such as impedance). That is, for example, due to differing impedanceprofiles of power sources, a first power source 56 having a relativelyhigher impedance than a second power source 56 may begin top-off period88 relatively earlier in the charging session than the second powersource 56, regardless of the state of charge of the power source 56.

FIG. 6 is a graph illustrating another example charging session havinganother top-off period, which may be performed according to aspects ofthis disclosure. For example, FIG. 6 generally illustrates an examplecharging session in which charging during a top-off period may be basedon battery capacity and battery state of charge, according to aspects ofthis disclosure. Although generally described as being performed bycomponents of IMD 34 (FIGS. 2, 3) and programmer 40 (FIGS. 2, 4) forpurposes of explanation, it should be understood that other implantablestimulators and programmers, such as implantable stimulator 4 andprogrammer 20 (FIG. 1), or a variety of other devices may also beconfigured to perform the charging session shown in FIG. 6. Moreover, inother examples, IMD 34 and/or programmer 40 may perform differentfunctions than those described with respect to FIG. 5 (e.g., IMD 34 mayperform functions attributed to programmer 40, and vice versa).

As with the example shown in FIG. 5, the example graph shown in FIG. 6tracks a charging current 90, a voltage 92, and a state of charge 94throughout a charging session of power source 56. As shown in FIG. 6,programmer 40 applies a charging current 90 to power source 56 during aninitial charging period 96, e.g., via inductive coils and suitablecharging circuitry within programmer 40 and within IMD 34. In theexample shown in FIG. 6, programmer 40 applies a current that, uponinductive transfer, induces a maximum charging current of approximately80 milliamps (mA) during the initial charging period 96. In otherexamples, the maximum charging current may vary depending on, forexample, the capabilities of power source 56, the capabilities ofprogrammer 40, or other factors.

The example shown in FIG. 6 includes a top-off period 98 based on acapacity of power source 56 and the state of charge 94 of the powersource. Top-off period 98 may be initiated by IMD 34 or programmer 40.

In some examples, IMD 34 may measure state of charge 94 using coulombcounter 62. For example, as noted above with respect to FIG. 3, coulombcounter 62 may be positioned at the output of power source 56 formeasuring the amount of charge, coulombs, or current being delivered tostimulation generator 49 during therapy and/or the amount of charge,coulombs, or current being received by power source 56 during a chargingsession. For example, coulomb counter 62 may count each unit of chargethat is discharged by power source 56, while also counting each unit ofcharge that is received by power source 56 during charging.

Accordingly, according to aspects of this disclosure, IMD 34 may trackthe state of charge 94 of power source 56 relative to the capacity ofpower source 56. Thus, IMD 34 may estimate or otherwise recalibrate thecapacity of power source 56 after each charging session. For example,IMD 34 may estimate the capacity of power source 56 for a currentcharging session based on an amount of charge applied to power source 56during a previous charging session.

In some examples, coulomb counter 62 of IMD 34 may track an amount ofcharge that is expended by power source 56, as well as an amount ofcharge that is stored by power source 56 during a charging session,assuming power source 56 is fully charged during each charging session.Accordingly, IMD 34 may estimate a new battery capacity after eachcharging session, based on the amount of charge that was expended duringtherapy versus the amount of charge that was applied during the chargingsession. In this way, IMD 34 may adjust the capacity of power source 56as power source 56 degrades over time and is able to store relativelyless charge. Other methods may also be used to determine the capacity ofpower source 56.

In the example shown in FIG. 6, IMD 34 begins top-off period 98 uponstate of charge 94 reaching approximately 40 mAh, or approximately 40%of charge for a power source 56 having a capacity of 100 mAh. Forexample, as noted above, a charging top-off period typically occursduring the end of a charging session, and may help to charge powersource 56 to full capacity without increasing the voltage of powersource 56 beyond a desirable level. In the example shown in FIG. 6, thetop-off may be triggered based on the state of charge of power source56.

While FIG. 6 illustrates IMD 34 beginning top-off period 98 atapproximately 40% state of charge 94, in other examples, IMD 34 maybegin top-off period 98 earlier (e.g., 35% of capacity, 30% of capacity,25% of capacity, and the like) or later (e.g., 50%, 60%, 80%, and thelike) during the charging session. The point at which IMD 34 beginstop-off period 98 may vary based on, for example, theconfiguration/capability of power source 56, the age of power source 56,the capability of programmer 40, or other factors. For example, as theage of power source 56 increases, the impedance of power source 56 mayalso increase (e.g., a battery's impedance may rise over time).Accordingly, IMD 34 may begin top-off period 98 earlier (e.g., at alower state of charge 94) for an older power source 56 than for a newerpower source 56 in order to maintain voltage 92 at or below a voltagelimit (“max voltage”).

During top-off period 98, IMD 34 may reduce charging current 90 used tocharge power source 56 as state of charge 94 approaches full capacity(“charge complete”). In some examples, IMD 34 may incrementally reducecharging current 90 during top-off period 98 as state of charge 94approaches full capacity. That is, in the example shown in FIG. 6, IMD34 incrementally reduces (“steps down”) charging current 90 four times,with a period of constant charging current 90 between each step duringtop-off period 98. In some examples, the step sizes may be preprogrammedin IMD 34. In other examples, IMD 34 may adapt the step sized based on arate at which voltage 92 of power source 56 is rising, a rate at whichstate of charge 94 is rising, or other factors. For example, if voltage92 of power source 56 is rising relatively rapidly, IMD 34 may increasethe step size in an effort to slow the rise in voltage.

In other examples, IMD 34 may step down charging current more or lessfrequently than that shown in FIG. 6 (e.g., two steps, three steps, sixsteps, and the like). Additionally or alternatively, IMD 34 may stepdown charging current 90 more or less quickly than that shown in FIG. 6.That is, in another example, IMD 34 may transition between a highercharging current 90 and a lower charging current 90 immediately (e.g.,characterized by a vertical decline between steps) during top-off period98. In yet another example, IMD 34 may transition between a highercharging current 90 and a lower charging current 90 more slowly thanthat shown in FIG. 6 (e.g., characterized by a more gradual declinebetween steps) during top-off period 98. In still other examples, IMD 34may continually and gradually lower charging current 90 during top-offperiod 96 (e.g., without “steps”).

According to the techniques of this disclosure, IMD 34 may continuecharging power source 56 until a minimum charging current 90 is reached.The minimum charging current 90 may be the smallest current at which IMD34 is allowed to charge power source 56. Minimum charging current 90 maybe reached due to IMD 34 reducing charging current 90 as power source 56reaches a full charge (e.g., according to state of charge 94). As notedabove, reducing charging current 90 may help to maintain voltage 92 ator below a voltage limit. In other examples, another chargingtermination parameter may be used to indicate when charging is complete.

The minimum charging current may be predefined, or may be adapted basedon power source 56 (e.g., as described below with respect to FIGS.10-11). As shown in the example of FIG. 6, by reducing the chargingcurrent 90 based on state of charge 94 and the capacity of power source56, current shunting (e.g., an internal diversion of charging current toa resistive load) can be avoided. That is, for example, IMD 34 may lowercharging current 90 based on state of charge 94 and the capacity ofpower source 56 while maintaining voltage 92 equal to or lower than avoltage limit (e.g., “max voltage”). In the example shown in FIG. 6, themax voltage is not reached until power source 56 has completed charging(e.g., state of charge 94 reaches “charge complete”). Thus, according tothe techniques of this disclosure, IMD 34 may charge power source 56using the largest charging current possible while also avoiding currentshunting.

FIG. 7 is a flow chart illustrating an example method 100 of recharginga power source, according to aspects of this disclosure. Althoughgenerally described as being performed by components of IMD 34 (FIGS. 2,3) and programmer 40 (FIGS. 2, 4) for purposes of explanation, it shouldbe understood that other implantable stimulators, charging devices andprogrammers, such as implantable stimulator 4 and programmer 20 (FIG.1), or a variety of other devices may also be configured to perform thecharging session shown in FIG. 7. In addition, power source 56 isgenerally referred in the example method of FIG. 7 as a battery. Asnoted above, however, power source 56 may include one or more batteries,capacitors, supercapacitors, or any combination thereof

IMD 34 begins a top-off period of a charging session (102). In someexamples, as noted above, IMD 34 may begin the top-off period uponrecharging the battery to a certain amount (e.g., a state of charge ofthe battery reaching a predetermined level). For example, IMD 34 maybegin the top-off period upon the battery reaching 40% charged. In otherexamples, IMD 34 may begin the top-off period relatively later in thecharging session, e.g., upon the battery reaching 80% charged. The pointat which IMD 34 begins the top-off period may vary based on, forexample, the configuration/capability of the battery, the age of thebattery, the configuration/capability of programmer 40, or otherfactors.

During the top-off period, IMD 34 determines the battery capacity (104).In some examples, battery capacity for a current charging session may beestimated based on an amount of charge applied to the battery during aprevious charging session. For example, coulomb counter 62 of IMD 34 maytrack an amount of charge that is expended by IMD 34, as well as anamount of charge that is stored by the battery during a chargingsession, assuming the battery is fully charged during each chargingsession. Accordingly, IMD 34 may estimate a new battery capacity aftereach charging session, based on the amount of charge that was expendedversus the amount of charge that was applied during the chargingsession. In this way, IMD 34 may adjust the capacity of the battery asthe battery degrades and is able to store relatively less charge.

IMD 34 also determines the battery state of charge (106). For example,IMD 34 may determine the battery state of charge relative to the batterycapacity. In some examples, IMD 34 may use coulomb counter 62 todetermine the state of charge. For example, given a certain batterycapacity (e.g., an estimated battery capacity), IMD 34 may determine thestate of charge of the battery as charge is applied to the batteryrelative to the calculated battery capacity.

In some examples, according to aspects of this disclosure, IMD 34 mayreduce the charging current that is applied to the battery in responseto the state of charge of the battery approaching the battery capacity(108). In some examples, as described with respect to FIG. 6 above, IMD34 may reduce, or “step down” the charging current more than once duringthe top-off period. In other examples, IMD 34 may continually andgradually reduce the charging current during the top-off period. In anyevent, the charging current may be reduced such that a voltage of thebattery is maintained below some predefined voltage limit.

IMD 34 then determines whether the battery is fully charged (110). Insome examples, determining whether the battery is fully charged mayinclude determining whether a minimum charging current of the top-offperiod has been reached. In other examples, determining whether thebattery is fully charged may include determining whether a voltage limithas been reached. In other examples, determining whether the battery isfully charged may include counting a number of coulombs of charge (ormeasuring an amount of current) that has been applied to the batteryduring charging and determining whether the charge applied matches thebattery capacity. In other examples, some combination of these methodsmay be used to determine whether the battery is charged (e.g., a minimumcharging current is reached and a voltage limit is reached).

If IMD 34 determines that charging is complete (the “yes” branch of step110), IMD 34 may end the charging session (112). In some examples, anindication may be provided to patient 6 or a clinician, e.g., via userinterface 63. If IMD 34 determines that charging is not complete (the“no” branch of step 110), IMD 34 may continue determining batterycapacity (104), determining state of charge (106), and reducing thecharging current in response to the state of charge approaching thebattery capacity (108).

In some examples, the method shown and described with respect to FIG. 7may be carried out by IMD 34 (e.g., processor 50). However, in otherexamples, the method of FIG. 7 may be performed by programmer 40 (e.g.,processor 64). That is, for example, programmer 40 may be responsiblefor applying the appropriate charging current during the top-off period.In other examples, certain steps and/or functions of the method shown inFIG. 7 may be performed by both IMD 34 and programmer 40 in combination.

It should also be understood that the steps shown and described withrespect to FIG. 7 are provided as merely one example. That is, the stepsof the method of FIG. 7 need not necessarily be performed in the ordershown in FIG. 7, and fewer, additional, or alternative steps may beperformed.

FIG. 8 is a graph illustrating an example charging session having atop-off period. Although generally described as being performed bycomponents of IMD 34 (FIGS. 2, 3) and programmer 40 (FIGS. 2, 4) forpurposes of explanation, it should be understood that other implantablestimulators and programmers, such as implantable stimulator 4 andprogrammer 20 (FIG. 1), or a variety of other devices may also beconfigured to perform the charging session shown in FIG. 8. Moreover, inother examples, IMD 34 and/or programmer 40 may perform differentfunctions than those described with respect to FIG. 8 (e.g., IMD 34 mayperform functions attributed to programmer 40, and vice versa).

The example graph shown in FIG. 8 tracks a charging current 120 and astate of charge 122 throughout a charging session of power source 56.The example shown in FIG. 8 may be a charging session for a relativelynew 120 mAh power source 56. That is, the example shown in FIG. 8 may beassociated with a newly manufactured and/or implanted IMD 34. In theexample shown in FIG. 8, charging current 120 is measured in milliamps(mA) and state of charge 122 is measured in milliamp-hours (mAh). Inother examples, state of charge 122 may also be represented as apercentage of the battery capacity (e.g., 50% of full charge).

Programmer 40 applies a charging current 120 to power source 56 duringan initial charging period 124. During the initial charging period 124,programmer 40 may apply a maximum charging current to power source 56 inorder to charge power source 56 as quickly as possible without damagingpower source 56. In the example shown in FIG. 8, programmer 40 applies amaximum charging current of approximately 100 milliamps (mA) during theinitial charging period 124. In other examples, the maximum chargingcurrent may vary depending on, for example, the capabilities of powersource 56, the capabilities of programmer 40, or other factors.

In the example shown in FIG. 8, IMD 34 reduces a charging current 120during top-off period 126. IMD 34 may reduce charging current 120 due toshunting (e.g., reaching a voltage limit, as described above) and/or inaccordance with a predefined charging algorithm. In the example of FIG.8, IMD 34 may continue to reduce charging current 120 during top-offperiod 126 until reaching a predetermined minimum charging current cutoff (“MIN CHARGE CURRENT CUT OFF”). That is, IMD 34 may continue tocharge power source 56 until a predefined, minimum charging current 120is reached. The minimum charging current may be the smallest current atwhich IMD 34 is allowed to charge power source 56. For example, it maynot be beneficial to charge power source 56 using a charging currentthat is lower than the minimum charging current, as power source 56 maynot be effectively charged, the charging session may take too long, andthe like.

The minimum charging current cut off may be initially set (e.g., at thetime of manufacture or implantation) so that the minimum chargingcurrent cut off is reached at the same time that power source 56 reachesfull charge. In the example shown in FIG. 8, power source 56 reaches afull charge (e.g., assuming a 120 mAh battery) at a minimum chargingcurrent cut off of approximately 30 mA. In addition, power source 56reaches full charge after a charging session of approximately 80 and 85minutes. The predefined minimum charging current cut off shown in FIG. 8may not be adaptive to changing conditions of power source 56, asdescribed in greater detail with respect to FIGS. 9-11 below.

FIG. 9 is a graph illustrating an example charging session for powersource 56 having the same predetermined minimum charging current cut offas that shown in FIG. 8 (e.g., approximately 30 mA). The example of FIG.9, however, may represent a relatively older power source 56 that hashigher impedance than that associated with FIG. 8. That is, in someexamples, the impedance of power source 56 may increase over time. Forexample, the impedance of power source 56 may increase due to naturaldegradation of power source 56 (e.g., associated with discharging andcharging cycles) and/or other wear or damage to power source 56.

The higher impedance of power source 56 may cause power source 56 toreach a voltage limit (e.g., due to IMD 34 shunting current) earlier inthe charging session than the lower impedance power source 56.Accordingly, IMD 34 may reduce charging current 120 during top-offperiod 126 shown in FIG. 9 relatively sooner than the lower impedancepower source 56 shown in FIG. 8. In addition, the minimum chargingcurrent cut off (e.g., 30 mA) is achieved in a relatively shorter amountof time. That is, where the example shown in FIG. 8 reaches the minimumcharging current cut off in approximately 80 to 85 minutes, the exampleshown in FIG. 9 reaches the minimum charging current cut off inapproximately 50 minutes. Accordingly, the state of charge 122 that isachieved in power source 56 having the higher impedance is less (FIG. 9)than that achieved in power source 56 having the lower impedance (FIG.10). For example, the state of charge is approximately 70 mAh in FIG. 9versus approximately 120 mAh in FIG. 8.

FIG. 10 is a graph illustrating an example charging session having atop-off period that includes a minimum charging current cut off, whichmay be adapted to account for changing battery conditions in accordancewith the techniques of this disclosure. That is, the example chargingsession shown in FIG. 10 may be associated with the same high impedancepower source 56 described with respect to FIG. 9. In the example of FIG.10, however, the minimum charging current cut off has been adapted toaccount for the higher impedance, as described in greater detail below.

Although generally described as being performed by components of IMD 34(FIGS. 2, 3) and programmer 40 (FIGS. 2, 4) for purposes of explanation,it should be understood that other implantable stimulators, chargingdevices and programmers, such as implantable stimulator 4 and programmer20 (FIG. 1), or a variety of other devices may also be configured toperform the charging session shown in FIG. 10.

In the example of FIG. 10, programmer 40 applies a charging current 140to power source 56 during an initial charging period 144. During theinitial charging period 144, programmer 40 may apply a maximum chargingcurrent to power source 56 in order to charge power source 56 as quicklyas possible without damaging power source 56. In the example shown inFIG. 10, programmer 20 applies a maximum charging current ofapproximately 100 milliamps (mA) during the initial charging period 144.In other examples, the maximum charging current may vary depending on,for example, the capabilities of power source 56, the capabilities ofprogrammer 20, or other factors.

The graph shown in FIG. 10 may be associated with the same relativelyhigh impedance power source associated with FIG. 9. That is, IMD 34 mayreduce charging current 140 during top-off period 146. As shown in FIG.10, however, implantable stimulator may reduce a minimum charge currentcut off (“MIN CHARGE CURRENT CUT OFF”) in accordance with the techniquesof this disclosure.

For example, IMD 34 may initially determine a duration of a previouscharging session, as well as an amount of charge that was administeredto power source 56 during the previous charging session. IMD 34 maythen, according to the techniques of this disclosure, determine thetop-off charging current cut off based on the duration of the previouscharging session. For example, IMD 34 may compare the duration of theprevious charging session to a predefined target duration. If theduration of the previous charging session was shorter than the targetduration (e.g., power source 56 completed charging in less time than thetarget duration), IMD 34 may allow power source 56 to be charged duringtop-off for a longer duration using a lower minimum charging current cutoff. Alternatively, if the duration of the previous charging session waslonger than the target duration, IMD 34 may increase the minimumcharging current cut off

Additionally or alternatively, according to aspects of this disclosure,IMD 34 may determine the top-off charging current cut off based on anamount of charge administered during the previous charging session. Forexample, if the amount of charge delivered during the previous chargingsession does not charge power source 56 to full or nearly full capacity,IMD 34 may lower the minimum charging current cut off. Lowering theminimum charging current cut off may increase the duration of thetop-off period and allow more charge to be administered to power source56.

In other examples, IMD 34 may implement a balancing approach thatconsiders both the charge administered during the first charging sessionand the target duration. For example, IMD 34 may attempt to maximize anamount of charge administered to power source 56 while still maintainingthe charging session to within a predetermined time range.

In the example shown in FIG. 10, IMD 34 lowers the minimum chargingcurrent cut off from approximately 40 mA (FIG. 9) to approximately 10mA. As shown in the example of FIG. 10, reducing the minimum chargingcurrent cut off results in an increased charging duration, as well as anincrease in the amount of charge that is administered to power source 56during charging. For example, the charging session shown in FIG. 10lasts for approximately 80 minutes while charging power source toapproximately 80 to 90 mAh of charge. In contrast, the charging sessionshown in FIG. 9 lasts for only approximately 50 minutes while chargingpower source to approximately 70 mAh of charge.

Adapting the minimum charging current cut off for a current chargingsession based on how much charge was put into power source 56 during aprevious charging session and a duration of the previous rechargesession allows IMD 34 to maintain the current charging session to apredetermined (target) time, while also cutting off the currentrecharging session if the charging current being applied to power source56 is lower than a certain amount.

While the examples shown in FIGS. 8-10 are described with respect to aminimum charging current cut off, it should be understood that, in otherexamples, other charging termination parameters may be used to end acharging session. For example, a power cut off value may be used to enda charging session. The power level cut off may be based on a poweroutput of programmer 40 (or another charging device). Thus, programmer40 may continue charging power source 56 until reaching the power levelcut off, which may be a minimum power level that may be used to chargepower source 56. In this example, according to the techniques of thisdisclosure, IMD 34 and/or programmer 40 may adapt the power level cutvalue off based on the duration of the previous charging session and/oran amount of charge administered during the previous charging session.In still other examples, other charging termination parameters may beused, such as other parameters that indicate when a battery has reachedfull charge.

FIG. 11 is a flow chart illustrating an example method 140 of recharginga power source, according to aspects of this disclosure. Althoughgenerally described as being performed by components of IMD 34 (FIGS. 2,3) and programmer 40 (FIGS. 2, 4) for purposes of explanation, it shouldbe understood that other implantable stimulators and programmers, suchas implantable stimulator 4 and programmer 20 (FIG. 1), or a variety ofother devices may also be configured to perform the charging sessionshown in FIG. 11. In addition, power source 56 is generally referred inthe example method of FIG. 11 as a battery. As noted above, however,power source 56 may include one or more batteries, capacitors,supercapacitors, or any combination thereof.

IMD 34 recharges the battery during a first charging session untilreaching a predetermined charging termination parameter value (aparameter value that, when attained, causes IMD 34 and/or programmer 40to terminate the charging session) (142). The charging terminationparameter may include, for example, a minimum charging current cut off,a power level cut off, a state of charge of the battery, and the like.For example, given a certain battery capacity (e.g., power capacity),IMD 34 may recharge the battery until a device responsible for chargingthe battery (e.g., programmer 40) lowers to a power level that occurswhen the battery is approximately charged to cull capacity. Othercharging termination parameters are also possible. As described withrespect to FIGS. 8-10, with respect to the minimum charging current cutoff example, the minimum charging current cut off may initially be setto a predetermined value that allows the battery to be fully charged.

IMD 34 then determines the duration of the first charging session (144).The duration of the first charging session may be determined, forexample, by timing the charging session with a timer that begins when acharge is first applied to the battery and ends upon reaching thecharging current cut off (e.g., ending the charging session). In someexamples, as described in greater detail below, IMD 34 may alsodetermine an amount of charge administered to the battery during thefirst charging session. The amount of charge administered to the batterymay be determined, for example, using coulomb counter 62. That is,coulomb counter 62 may track a number of coulombs administered to thebattery during charging. In other examples, IMD 34 may determine anamount of current that is administered to the battery during chargingusing coulomb counter 62.

IMD 34 then determines a new charging termination parameter value forthe next charging session based on the duration of the previous chargingsession (146). For example, according to aspects of this disclosure, IMD34 may compare the duration of the previous charging session to apredefined, target duration. In some examples, the target rechargingtime may be static and may be programmed at the time of manufacture ofIMD 34. In other examples, the target recharging time may be determinedby patient 36 and/or clinician. In such examples, the target rechargingtime may be input via programmer 40. In an example, the predefinedtarget duration may be approximately one hour, although longer orshorter durations may be used depending, for example, on theconfiguration/capability of the battery, the configuration/capability ofprogrammer 40, or other factors. For example, a relatively smallerbattery may be associated with a relatively shorter target chargingduration.

According to aspects of this disclosure, if the duration of the previouscharging session was shorter than the target duration (e.g., the batteryfinished charging in less time than the target duration), IMD 34 mayalter the charging termination parameter value in a way that increasesthe duration of the next charging session. For example, in examples inwhich the charging termination parameter is a minimum charging currentcut off, IMD 34 may reduce the minimum charging current cut off value.That is, for example, IMD 34 may allow the battery to be charged alonger duration using a lower minimum charging current cut off.Alternatively, if the duration of the previous charging session waslonger than the target duration, IMD 34 may increase the minimumcharging current cut off. Altering the charging termination parameter inthis way may be used primarily in situations in which the battery isbeing charged to a full charge from a fully depleted state.

According to aspects of this disclosure, IMD 34 may also consider thecapacity of the battery in situations in which the battery is not fullydischarged before beginning a charging session when determining the newcharging termination parameter value. In an example for purposes ofillustration, a user may begin charging IMD 34 despite the batteryhaving approximately 50% charge remaining The charging session may lastapproximately 20 minutes to charge the battery to a full charge.Accordingly, IMD 34 may determine that the next charging sessionrequires 40 minutes to charge the battery from fully discharged to fullycharged. Given a target recharge time of one hour, IMD 34 may alter thecharging termination parameter value for the next charging session in aneffort to increase the charging time to one hour. That is, for example,IMD 34 may lower a minimum charging current cut off value to increasethe duration of the next charging session.

Additionally or alternatively, according to some aspects of thisdisclosure, IMD 34 may also determine the new charging terminationparameter value based on a comparison of the amount of chargeadministered to the battery for the previous charging session to acapacity of the battery. In some examples, the battery capacity may be astatic, predetermined value. In other examples, the battery capacity maybe estimated based on an amount of charge applied to the battery, asdescribed above. If the amount of charge delivered during the previouscharging session does not charge the battery to full or nearly fullcapacity, IMD 34 may alter the predetermined charging terminationparameter value accordingly. For example, IMD 34 may lower a minimumcharging current cut off. Lowering the minimum charging current cut offmay increase the duration of the top-off period and allow more charge tobe administered to the battery. For example, by lowering the minimumcharging current cut off, the battery can be maintained below a voltagelimit (e.g., a charging voltage limit) for a longer period of time, andadditional charge can be administered to the battery.

In some examples, IMD 34 may implement a balancing approach thatconsiders both the charge administered during the first charging sessionand the target duration. In an example for purposes of illustration,assume the battery has a 110 mAh capacity, a first charging session endswith a 100 mAh state of charge, a 40 mA minimum charging current cutoff, and a charging duration of approximately 50 minutes (given a targetof 60 minutes). For the next charging session, IMD 34 may adjust acharging termination parameter value (e.g., a minimum charging currentcut off value, or a value of another charging termination parameterdescribed above) until the one hour target duration is achieved.Achieving the target charging duration may result in a 95 mAh state ofcharge and a minimum charging current cut off of approximately 30 mA. Ina subsequent charging session, IMD 34 may adjust the minimum chargingcurrent cut off to achieve the target charging duration, to achieve afull state of charge, or a balance of the two (e.g., a charging timefrom 50 to 70 minutes and a state of charge of 90 to 110 mAh).

In this way, IMD 34 can account for changing battery impedance, whilealso maintaining a target charging time. For example, as noted above,higher battery impedance may cause a battery to reach a voltage limitduring charging more quickly (e.g., relative to a battery having lowerimpedance). In addition, higher battery impedance may cause the batteryto reach the charging termination parameter value more quickly,resulting in less charge being administered to the battery. According tothe techniques of this disclosure, the charging termination parametervalue may be adjusted to account for changing impedance and allow thebattery to charge for a longer duration, while also ensuring that theduration of the charging session is not extended beyond a targetduration.

After determining the new charging termination parameter value (146),IMD 34 may charge the battery during the next charging session until thenew charging termination parameter value is reached (148). In someexamples, the method shown and described with respect to FIG. 11 may becarried out by IMD 34 (e.g., processor 50). However, as noted withrespect to FIGS. 3 and 4 above, the method of FIG. 11 may also beperformed by programmer 40 (e.g., processor 64). That is, for example,programmer 40 may be responsible for applying the appropriate chargingtermination parameter value. In other examples, certain steps and/orfunctions of the method shown in FIG. 11 may be performed by both IMD 34and programmer 40 in combination.

It should also be understood that the steps shown and described withrespect to FIG. 11 are provided as merely one example. That is, thesteps of the method of FIG. 11 need not necessarily be performed in theorder shown in FIG. 11, and fewer, additional, or alternative steps maybe performed.

The techniques described in this disclosure, including those attributedto processors 50, 62, coulomb counter 62, or various constituentcomponents, may be implemented, at least in part, in hardware, software,firmware or any combination thereof. For example, various aspects of thetechniques may be implemented within one or more processors, e.g.,processors 50, 62, including one or more microprocessors, digital signalprocessors (DSPs), application specific integrated circuits (ASICs),field programmable gate arrays (FPGAs), or any other equivalentintegrated or discrete logic circuitry, as well as any combinations ofsuch components, embodied in programmers, such as physician or patientprogrammers, stimulators, image processing devices or other devices. Theterm “processor” or “processing circuitry” may generally refer to any ofthe foregoing logic circuitry, alone or in combination with other logiccircuitry, or any other equivalent circuitry.

Such hardware, software, firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as random access memory(RAM), read-only memory (ROM), non-volatile random access memory(NVRAM), electrically erasable programmable read-only memory (EEPROM),FLASH memory, magnetic data storage media, optical data storage media,or the like. The instructions may be executed to support one or moreaspects of the functionality described in this disclosure.

In general, the techniques described in this disclosure can be appliedto devices that are powered by one or more power sources such asbatteries or capacitors. The techniques may be applied to medicaldevices such implantable medical devices configured to deliverneurostimulation or other electrical stimulation therapy via implantedelectrode arrays, carried by leads or otherwise, located proximate tothe spinal cord, pelvic nerves, peripheral nerves, the stomach or othergastrointestinal organs, or within the brain of a patient. Thetechniques described in this disclosure can be applied to medicaldevices that may not include electrodes to provide electricalstimulation. For examples, the techniques described in this disclosurecan be applied to medical devices that provide medication in accordancewith a delivery schedule. The techniques described in this disclosuremay also be applied to medical devices that are external to the patient,as well as medical devices that used to program other medical devices.The techniques described in this disclosure may also be applied tonon-medical devices such as laptop computers, gaming counsels, mobilephones, personal digital assistants (PDAs), and other such devices.

Many aspects of the disclosure have been described. Variousmodifications may be made without departing from the scope of theclaims. These and other aspects are within the scope of the followingclaims.

The invention claimed is:
 1. A method of charging a battery, the methodcomprising: determining a capacity of the battery during a chargingsession for charging the battery; determining a state of charge of thebattery during charging session; determining, based on the capacity andthe state of charge, a charging current for charging the batter during atop-off session that occurs prior to the end of the charging session andthat comprises less than all of the charging session, whereindetermining the charging current comprises reducing, prior to the end ofthe charging session, the charging current to one or more non-zerocurrent levels in response to the state of charge of the batterychanging with respect to the capacity of the batter; and charging thebattery with the determined charging current during the top-off session.2. The method of claim 1, further comprising determining a voltage limitfor the battery, and wherein charging the battery comprises maintaininga voltage of the battery below the voltage limit.
 3. The method of claim1, wherein determining the capacity of the battery comprises determininga maximum number of storable amp-hours of the battery.
 4. The method ofclaim 3, wherein determining the state of charge of the batterycomprises determining a difference between a current number of storedamp-hours of the battery and the maximum number of storable amp-hours ofthe battery.
 5. The method of claim 4, further comprising measuring thecurrent number of stored amp-hours using a coulomb counter.
 6. Themethod of claim 3, wherein the maximum number of storable amp-hours ofthe battery is an estimated value, and further comprising decreasing theestimated value after fully charging the battery based on a number ofamp-hours needed to fully charge the battery.
 7. The method of claim 1,wherein the top-off session comprises charging of a final twenty percentof the capacity of the battery.
 8. The method of claim 1, whereincharging the battery comprises a transcutaneous inductive transfer ofcharge to the battery.
 9. The method of claim 1, wherein the batteryresides within an implantable medical device, wherein charging thebattery comprises charging the battery with a medical device charger.10. The method of claim 1, further comprising stopping charging thebattery upon reducing the charging current to a minimum non-zerocharging current cut off.
 11. An implantable medical device (IMD)comprising: a battery configured to power the IMD; a memory storinginstructions; one or more processors configured to execute theinstructions, wherein upon execution of the instructions, the one ormore processors cause: determining a capacity of the battery during acharging session for charging the battery; determining a state of chargeof the battery during the charging session; determining, based on thecapacity and the state of charge, a charging current for charging thebattery during a top-off session that occurs prior to the end of thecharging session and that comprises less than all of the chargingsession, wherein determining the charging current comprises reducing,prior to the end of the charging session, the charging current to one ormore non-zero current levels in response to the state of charge of thebattery changing with respect to the capacity of the battery; andcharging the battery with the determined charging current during thetop-off session.
 12. The IMD of claim 11, wherein upon execution of theinstructions, the one or more processors further cause: determining avoltage limit for the battery, and wherein charging the batterycomprises maintaining a voltage of the battery below the voltage limit.13. The IMD of claim 11, wherein determining the capacity of the batterycomprises determining a maximum number of storable amp-hours of thebattery.
 14. The IMD of claim 13, wherein determining the state ofcharge of the battery comprises determining a difference between acurrent number of stored amp-hours of the battery and the maximum numberof storable amp-hours of the battery.
 15. The IMD of claim 14, furthercomprising a coulomb counter configured to measure the current number ofstored amp-hours.
 16. The IMD of claim 13, wherein the maximum number ofstorable amp-hours of the battery is an estimated value, and whereinupon execution of the instructions, the one or more processors furthercause: decreasing the estimated value after fully charging the batterybased on a number of amp-hours needed to fully charge the battery. 17.The IMD of claim 11, wherein the top-off session comprises charging of afinal twenty percent of the capacity of the battery.
 18. The IMD ofclaim 11, further comprising a charging module configured to inductivelytransfer charge to the battery transcutaneously.
 19. The IMD of claim11, wherein upon execution of the instructions, the one or moreprocessors further cause: stopping charging the battery upon reducingthe charging current to a minimum non-zero charging current cut off. 20.An apparatus for charging a battery, the apparatus comprising: means fordetermining a capacity of the battery during a charging session forcharging the battery; means for determining a state of charge of thebattery during the charging session; means for determining, based on thecapacity and the state of charge, a charging current for charging thebattery during a top-off session that occurs prior to the end of thecharging session and that comprises less than all of the chargingsession, wherein determining the charging current comprises reducing,prior to the end of the charging session, the charging current to one ormore non-zero current levels in response to the state of charge of thebattery changing with respect to the capacity of the battery; and meansfor charging the battery with the determined charging current during thetop-off session.
 21. The apparatus of claim 20, further comprising meansfor determining a voltage limit for the battery, and wherein the meansfor charging the battery comprises means for maintaining a voltage ofthe battery below the voltage limit.
 22. The apparatus of claim 20,wherein the means for determining the capacity of the battery comprisesmeans for determining a maximum number of storable amp-hours of thebattery.
 23. The apparatus of claim 22, wherein the means fordetermining the state of charge of the battery comprises means fordetermining a difference between a current number of stored amp-hours ofthe battery and the maximum number of storable amp-hours of the battery.24. The apparatus of claim 22, wherein the maximum number of storableamp-hours of the battery is an estimated value, and further comprisingmeans for decreasing the estimated value after fully charging thebattery based on a number of amp-hours needed to fully charge thebattery.
 25. The apparatus of claim 20, wherein the top-off sessioncomprises charging of a final twenty percent of the capacity of thebattery.
 26. The apparatus of claim 20, further comprising means forinductively transferring charge to the battery transcutaneously.
 27. Theapparatus of claim 20, further comprising means for stopping chargingthe battery upon reducing the charging current to a minimum non-zerocharging current cut off.